The present invention relates to a planar waveguide dispersion compensator for an optical signal, and a method for compensating for dispersion in an optical signal.
Glass fiber pulse code modulation (PCM) transmission systems are known to suffer from chromatic (wavelength dependent) dispersion. Such dispersion leads to optical signals propagating along a fiber being subject to delays in their propagation time along the fiber which depend on their wavelength. This variable delay generates several problems in optical communications networks. As transmission rates increase in digital optical communications networks cheap, reliable and efficient means to implement dispersion compensation and to control the pulse profile of an optical signal during transmission through optical media are becoming highly desirable.
The theoretical approach to preventing spread in a digital signal during transmission involves compensating for the variations in phase that arise from a frequency dependent group velocity in the transmission system.
Two ways a system may compensate for dispersion are adding a length of line, for example an additional length of optical waveguide, of opposite dispersion characteristics to the previous portion of the line or applying a suitable phase-versus-frequency characteristic to the signal. Consider the case where a spectral component of a signal propagating along line 1 of length z1 has angular frequency xcfx89. The spectral component has a propagation constant xcex21 along line 1. Along an additional length of line, line 2 of length z2, the spectral component has a propagation constant xcex22. Either propagation constant xcex21,xcex22 may be frequency dependent. If the initial, arbitrary, phase is "PHgr"0, then the phase at output is "PHgr"1=xcfx89t+Ø0xe2x88x92xcex21z1xe2x88x92xcex22z2.
The change of phase at a given frequency deviation xcex4xcfx89 from the center frequency is given by {txe2x88x92(dxcex21/dxcfx89)z1xe2x88x92(dxcex22/dxcfx89)z2xe2x88x92xcex22(dz2/dxcfx89)}xcex4xcfx89. To prevent distortion of the signal, the phase variation should remain zero over the whole range of frequencies contained within it. As the dxcex2/dxcfx89 and dz/dxcfx89 terms can vary over the frequency range, it is necessary that the second derivative with respect to frequency is also zero giving:                               (                                    d              2                        ⁢                                          β                1                            /              d                        ⁢                          xe2x80x83                        ⁢                          ω              2                                )                          (          1          )                    ⁢              z        1              +                            (                                    d              2                        ⁢                                          β                2                            /              d                        ⁢                          xe2x80x83                        ⁢                          ω              2                                )                          (          2          )                    ⁢              z        2              +          2      ⁢                        (                      d            ⁢                          xe2x80x83                        ⁢                                          β                2                            /              d                        ⁢                          xe2x80x83                        ⁢            ω                    )                          (          3          )                    ⁢              (                  d          ⁢                      xe2x80x83                    ⁢                                    z              2                        /            d                    ⁢                      xe2x80x83                    ⁢          ω                )              +                  β        2            ⁢                        (                                    d              2                        ⁢                                          z                2                            /              d                        ⁢                          xe2x80x83                        ⁢                          ω              2                                )                          (          4          )                      =  0
The above equation shows three ways that are available for compensating group delay distortion in a fixed length z1 of line 1 represented by term (1). Firstly, term (2), can provide compensation by adding line 2 of length z2 of opposite group velocity dispersion. Secondly, term (3) can provide compensation when the length z2 of line 2 is linearly dependent on the frequency. Thirdly, term (4) can provide compensation when the length z2 of line 2 is a strongly quadratic function of frequency and dominates term (3). Inducing a sufficiently negative dispersion of group velocity to enable an optical pulse to remain unchanged as it propagates requires consideration of a number of factors, particularly in a planar waveguide environment. Although polymer materials can provide a negative dispersion of group velocity, such materials are generally considered unsuitable for pulse reforming due to size constraints in a planar waveguide device. An optical pulse needs to have a relatively long propagation path within the polymer material to ensure a sufficiently negative group delay dispersion is induced.
Another way to induce a negative group velocity dispersion for a signal is to linearly change the path-length of each component signal of a pulse to induce a sufficient relative change in phase with respect to the relative wavelength difference between the component signals. This is described by term (3) in the equation and can be achieved in non planar optical environments for example, by using an adjustable chirped grating.
Conventional dispersion compensators using techniques such as stretchable chirped fibre gratings to alter the refractive index of the fibres implementing the grating are complex, expensive, and are subject to fatigue.
One object of the present invention seeks to obviate or mitigate the above problems by providing a dispersion compensator for an optical signal. Another object of the present invention seeks to provide a method of compensating for dispersion in an optical signal. Another object of the invention seeks to provide an optical component including a dispersion compensator. Another object of the invention seeks to provide a node for an optical network including a dispersion compensator. Another object of the invention seeks to provide an optical transmission system including a dispersion compensator. Yet another object of the invention seeks to provide a planer waveguide strip lens for use in a dispersion compensator. Yet another object of the invention seeks to provide a composite strip lens for use in a dispersion compensator.
One aspect of the invention provides a dispersion compensator for an optical signal comprising:
an arrayed waveguide grating having a number M of waveguides, the arrayed waveguide grating decomposing the optical signal into N component signals each having a separation wavelength xcex4xcex from an adjacent component signal;
at least one path-length adjuster varying the path-length of at least one of the N component signals to induce a phase shift xcex94xcfx86 between the initial phase of each component signal in the AWG waveguides and the final phase of each component signal output by the AWG waveguides; and
a recombiner to re-combine the phase-shifted component signals into a re-combined signal, wherein the phase shift xcex94xcfx86 of each component signal is selected to adjust at least one characteristic of the optical signal in the re-combined signal.
The dispersion compensator may further include an M:N coupler, wherein the arrayed waveguide grating is connected to the M:N coupler such each of the N component signals is carried along one of N waveguides.
The component signal separation wavelength xcex4xcex multiplied by the number of waveguides N preferably equals the bandwidth xcex94xcex of the optical signal.
At least one path-adjuster may comprise at least one lens having a refractive index which is capable of differing from the refractive index of a waveguide along which a component signal is propagating.
At least one path-adjuster preferably comprises at least one strip lens having a refractive index which is capable of differing from the refractive index of a waveguide along which a component signal is propagating, and wherein at least one strip lens is thicker at either end than in a middle portion.
Preferably, at least one characteristic is a group delay of the optical signal.
Preferably, the phase shift xcex94xcfx86 of each component signal is a quadratic function of the wavelength of each component signal.
At least one characteristic of the optical signal adjusted is preferably a width of a pulse profile of the optical signal.
The phase shift xcex94xcfx86 of each component signal is preferably determined to induce an appropriate dispersion compensating group delay for the re-combined signal.
Preferably, the recombiner comprises: a reflector capable of reflecting the phase shifted component signals; the reflector being provided so that the phase shifted component signals return along their incident paths.
For example, the reflector may be a mirror or mirror or a partially silvered mirror(s).
The recombiner may include a N:M coupler; an arrayed waveguide having a number M of waveguides, and M:1 coupler provided to combine the phase shifted component signals into a single signal.
The path length adjuster may have at least one thermal characteristic affecting the path-length of at least one component signal, and the dispersion compensator may further include thermal control means controlling the path adjustment means.
The dispersion compensator may further include a polarisation adjuster to adjust the polarisation of the component signals.
The dispersion compensator thus advantageously enables an optical signal which has undergone dispersion to be narrowed within an optical medium. By providing such a dispersion compensator as a planar waveguide device, the dispersion compensator is compact and easily integrated into optical components.
A second aspect of the invention seeks to provide a method of compensating for dispersion in an optical signal comprising the steps of:
decomposing the optical signal into component signals which differ from each other by a fractional wavelength xcex4xcex;
adjusting the phase of each component signal by an induced phase shift xcex94xcfx86; and
re-combining each component signals into a re-combined signal, wherein the phase shift xcex94xcfx86 is selected to adjust at least one characteristic of the optical signal in the re-combined signal.
The method may further comprise the step of selecting the induced phase shift xcex94xcfx86 to determine a group delay dispersion of the re-combined signal.
Preferably, the method further includes the step of selecting the phase shift xcex94xcfx86 to provide a different group delay dispersion for the re-combined signal to the initial group delay dispersion of the optical signal.
Preferably, the method further includes the step of selecting the phase shift xcex94xcfx86 of each component signal to induce zero group delay dispersion in the re-combined signal.
The method may further include the step of selecting the phase shift xcex94xcfx86 of each component signal to be a function of the wavelength of each component signal.
The method may further include the step of selecting the phase shift for each component signal to be a quadratic function of the wavelength of each component signal.
The method may further include the step of selecting the phase shift of each component signal to adjust the width of a pulse profile of the optical signal.
The method may further include the step of adjusting the phase of each component signal using thermally dependent path-length adjusting means to adjust the relative path-length of the component signals.
The method may further include the step of adjusting the polarisation of each component signal.
A third aspect of the invention seeks to provide an optical component including a dispersion compensator according to a first aspect of the invention.
A fourth aspect of the invention seeks to provide a node for an optical network including a dispersion compensator according to a first aspect of the invention.
A fifth aspect of the invention seeks to provide an optical transmission system including a dispersion compensator according to a first aspect of the invention.
A sixth aspect of the invention seeks to provide a planar waveguide dispersion compensator for an optical signal which applies a phase shift xcex94xcfx86 to the optical signal, where the phase shift xcex94xcfx86 is a function of the wavelength of the optical signal, and wherein the phase shift xcex94xcfx86 is selected to adjust at least one characteristic of the optical signal in the re-combined signal.
A seventh aspect of the invention seeks to provide a planar waveguide strip lens, the strip lens comprising: a middle portion of substantially uniform thickness; and at least one end portion substantially thicker than said middle portion. Preferably, at least one end portion is stepped.
An eighth aspect of the invention seeks to provide a planar waveguide composite lens comprising a plurality of strip lens, at least one strip lens comprising: a middle portion of substantially uniform thickness; and at least one end portion substantially thicker than said middle portion. Preferably, at least one end portion is stepped. Preferably, the composite lens has a substantially parabolic profile.
Any features of the above features may be suitably incorporated in any of the above aspects as would be apparent to a person skilled in the art. Moreover, terms such as adjuster are to be construed to include appropriate equivalents capable of acting as an adjuster as would be obvious to those skilled in the art. Similarly, terms such as re-combiner are to be construed to include appropriate equivalents capable of acting as a signal recombiner.
The invention thus provides a planar dispersion compensator for an optical signal. The compensator decomposes an inputted optical signal into N component signals separated by a fractional wavelength xcex4xcex. Each component signal has its path-length adjusted to induce a sufficient phase shift between input and output to change the group delay dispersion of the optical signal when recombined from each of the component signals. This behaviour is described by term (4) in the equation presented herein above. In this manner, pulse broadening can be compensated by selectively varying the induced phase shifts to produce the desired level of opposite group delay dispersion.
Advantageously, the dispersion compensation mechanism provides a means of inducing a group delay dispersion opposite to that of an optical signal in a relatively compact area. This is particularly advantageous in optical networks which carry traffic at high transmission rates. In any high-bit rate environment it is highly advantageous to be able to compensate signal dispersion in a reliable and compact manner.
By compensating for dispersion in the optical layer, both passive or active dispersion compensation can be implemented i.e. the amount of compensation may be predetermined (passive) or actively adjusted. Another advantage of the invention is that the invention can be implemented in a planar optical device.
The invention enables digital optical signal processing which comprises one or more instances of apparatus embodying the present invention, together with other additional apparatus.
By using the differential thermal response of different materials in a planar AWG, the mechanical strain/stress mechanisms such as stretchable chirped fibre gratings employ can be avoided.